Chicago in a broad area of seasonably mild air

Chicago is positioned under huge high pressure ridge aloft in an area of seasonably mild air, but changes loom ahead. A trough of low pressure over the Rockies is moving east, carrying cold air, with very cold/frigid air poised over west-central Canada sagging into the northern plains. The leading edge of cold air will reach our area Friday, just after warmth peaks here Thursday ahead of the approaching trough. Southern tip of very cold/frigid air reaches into our area this weekend and persists here next week.